There are many applications for frequency synthesisers that are capable of operating over a very wide frequency range. Instrumentation, millimetre wave and sub-millimetre wave, radar, communications systems, imaging systems, spectroscopy applications and general laboratory test equipment have requirements in this field. Existing ultra-wideband generators of frequencies in the range covered by the present invention all work by switching over from one type of frequency generator to another as the output frequency required goes out of the range of a particular generator type.
There is one type of synthesiser that works by combining two or more laser sources and presenting this combined signal to a photodetector. The photodetector output will comprise, amongst other components, fL1–fL2, where fL1 and fL2 are the respective frequencies of the two the laser sources. By a suitable choice of f1 and f2, any frequency can be produced that is within the bandwidth of the photodetector. There are problems with this technique. The linewidth of a typical laser can be several megahertz (MHz) wide, and combining two uncorrelated laser sources in a photodiode will produce an output signal that is too noisy for most practical purposes. Techniques exist1, 2 for ameliorating this by locking the two laser sources to a common reference signal such as, for example, another laser source so that much of the noise is correlated and so does not appear in the wanted output. Injection locked lasers would commonly be used as the laser sources as these are easily locked to a reference laser source.
These techniques can be used to provide an output frequency of low noise, but they suffer the disadvantage that the system output frequency is relatively inflexible, in that it can only be adjusted in increments of approximately a few hundred MHz minimum. For instance, if a laser with multiple frequency outputs is used as the reference source, then each of the injection locked lasers can be locked to a different output, but when the signals are combined in the photodetector, the output frequency step is limited to the difference between two successive outputs from the reference laser. If the number of frequency outputs per unit bandwidth from the reference is increased, so as to reduce the minimum step size, then it will be harder to keep each of the laser sources locked to one specific frequency line. Drift in the injection locked lasers will cause them to lock on to adjacent spectral lines if the lines are too close to each other.
The present invention overcomes these limitations, and also provides for very quick changes between desired frequencies.